Prostate cancer is the second leading cause of cancer deaths among males in the United States. While effective treatments exist for as long as the tumors remain sensitive to androgens, there are no good therapies available once evolution to an androgen-insensitive state occurs. An evaluation of factors which contribute to the growth of these later stage tumors is crucial to the development of new therapeutic strategies. Prolactin (PRL) is one such factor. Although large quantities of PRL are produced by the pituitary, PRL also serves as an autocrine growth factor in the normal human prostate and, as we have shown, this autocrine growth loop continues to be operative in cells representative of highly metastatic, androgen-independent cancers. Using a novel PRL growth antagonist developed in our laboratory, S179D PRL, we have demonstrated that blockade of the PRL autocrine growth loop reduces both the growth of well-established tumors and tumor initiation when androgen-independent cancer cells are grown in nude mice. This PRL growth antagonist therefore has the potential to be an important new therapeutic for late stage disease. The overall aim of the proposed project is to gain a fuller understanding of the molecular and cellular mechanisms underlying S179D PRL's antagonism of prostate tumor growth so that the full potential of this therapeutic can be realized. The specific aims are 1) to determine the optimal dose of S179D PRL, 2) to establish whether S179D PRL slows or halts growth or actually reduces tumor size, 3) to determine whether resistance can develop, and 4) to determine whether any aspect of the growth inhibition involves a) the promotion of apoptosis, or b) downregulation of the growth-promoting PRL autocrine loop, or c) upregulation of the short PRL receptor. In most experiments tumor size will be monitored by the amount of a secreted transfected gene product in the urine. In this way, individual nude mice can be assessed for their response to treatment. S179D PRL will be administered by Alzet minipumps implanted subcutaneously. Molecular and cellular effects on the tumors/tumor cells will be assessed by end-labeling for apoptosis, Northern, Western and microarray analysis for gene expression and immunoprecipitation, Western and radiolabel incorporation for signaling.